Estimate radiative power using Stefan–Boltzmann law accurately. Switch units, include surroundings, and compare cases easily. Perfect for labs, astronomy, HVAC, and materials design work.
The Stefan–Boltzmann law relates radiative power to absolute temperature. For a diffuse gray surface, the radiated power (absolute, to 0 K) is:
When the surface exchanges radiation with surroundings at temperature Tₑ, the net radiative power is:
| ε | Area | T | Tₑ | Radiated Power (W) | Net Power (W) |
|---|---|---|---|---|---|
| 0.90 | 0.50 m² | 500 K | 300 K | 1594.793 | 1388.108 |
| 0.95 | 2.00 ft² | 100 °C | 25 °C | 194.057 | 114.965 |
| 1.00 | 2500 cm² | 800 K | — | 2321.434 | — |
This tool estimates thermal radiation from a surface using the Stefan–Boltzmann law. It reports absolute radiated power (to a 0 K reference) and, when surroundings temperature is provided, net radiative power. Outputs include watts and kilowatts, plus radiant exitance in W/m² for quick comparison between materials and sizes.
The Stefan–Boltzmann constant is set to 5.670374419×10⁻⁸ W·m⁻²·K⁻⁴. This value links temperature to emitted radiant energy for an ideal blackbody. Real surfaces emit less, which is captured by emissivity ε between 0 and 1.
Radiation scales with T⁴, so small temperature increases can create large power changes. For example, doubling absolute temperature increases emission by 16×. This sensitivity is why radiation becomes important at high temperatures in furnaces, heat shields, and space hardware.
Emissivity depends on material, finish, and wavelength. Matte black coatings can be around 0.90–0.98. Oxidized metals often fall near 0.6–0.9. Polished metals can be much lower, sometimes 0.02–0.10. Use measured values when accuracy matters.
Absolute power uses P = εσAT⁴. Net power accounts for radiation received from surroundings: Pnet = εσA(T⁴ − Tₑ⁴). If T < Tₑ, net power becomes negative, indicating net radiant gain rather than loss.
Area may be entered in m², cm², mm², ft², or in² and is converted internally to m². Temperature can be entered in K, °C, or °F and is converted to Kelvin. This keeps the physics consistent while letting you work in the units used in your lab or design notes.
Consider ε = 0.90, A = 0.50 m², T = 500 K, and Tₑ = 300 K. The calculator reports about 1595 W radiated power and about 1388 W net loss. The difference is the radiation absorbed from the 300 K surroundings.
Results assume a diffuse gray surface and do not include view factors, partial enclosure effects, or spectral emissivity. For close surfaces, reflective cavities, or small view factors, use a radiative network model. Combine this tool with convection estimates to build a realistic heat-balance.
Emissivity is the fraction of blackbody radiation a surface emits at the same temperature. It ranges from 0 to 1 and depends on material, finish, and wavelength.
The Stefan–Boltzmann relation uses absolute temperature. Kelvin starts at absolute zero, so T⁴ remains physically meaningful. The calculator converts °C and °F to Kelvin automatically.
Use net power when the surface exchanges radiation with an environment at temperature Tₑ. It estimates the actual radiative heat loss or gain relative to surroundings.
Yes. If the surroundings are warmer than the surface (Tₑ > T), the term (T⁴ − Tₑ⁴) becomes negative, meaning the surface gains heat by radiation.
No. It only models radiative exchange. For a full thermal estimate, add convection and conduction terms separately and combine them in an energy balance.
Enter the effective radiating area that “sees” the environment. If only one side radiates significantly, use that side’s area; otherwise use the total exposed area.
Differences can come from inaccurate emissivity, changing surface temperature, geometry and view factors, reflections, or additional heat transfer modes. Use measured ε values and consider enclosure effects for higher accuracy.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.